The invention is particularly suited for an application in the field where the prime mover for the constant speed hydraulic drive is an internal combustion engine whose speed is throttle controlled and where the prime mover is a multi-purpose engine subject to variations in speed and loading imposed by the hydraulic drive and also to other loading as well. A particular application for this constant speed hydraulic drive is to drive a generator whose output frequency must be maintained constant. Loading of the generator may also be subject to variations.
Prior art servo control systems using feedback devices to control the output of the pump to maintain the speed of a hydraulic motor constant are inherently slower in response time and are subject to some oscillation and instability with respect to maintaining the speed of the hydraulic motor finitely constant. Servo feedback systems controlling the pump stroke to control motor speed encounter delay in response time due to compressibility of the hydraulic fluid. Motor acceleration when required is delayed because a portion of pump delivery is used to compress the fluid to be delivered to the hydraulic motor, so the servo commands the pump to even greater stroke. The reverse of this problem occurs during deceleration of the hydraulic motor. The servo feedback system in adjusting to overspeed and underspeed of the hydraulic motor encounters oscillations and instability of the speed of the hydraulic motor.